1. Field of the Invention
The invention relates to a method for detecting whether or not a display mode has to be switched, and more particularly to a method for detecting whether or not a display mode has to be switched, which is adopted in a liquid crystal display panel.
2. Description of the Related Art
Since the liquid crystal display (LCD) is advantageously thin in volume, light in weight and has low electromagnetic radiation, it is widely used recently. It is an important subject of the manufacturer as to how to reduce the cost of the liquid crystal display and increase the product competitiveness.
The conventional liquid crystal display panel is mainly composed of a top substrate, a bottom substrate, and a liquid crystal layer interposed therebetween. A common electrode is formed on a bottom surface of the top substrate, and a thin film transistor (TFT) for controlling a pixel electrode is formed on a top surface of the bottom substrate. Liquid crystal molecules in the liquid crystal layer change their tilts according to a voltage difference between the common electrode and the pixel electrode. The liquid crystal molecules with different tilts have different transmission rates with respect to a light ray, and enable display units on the liquid crystal display panel to display different brightness corresponding to different gray-scale values.
FIG. 1 is a graph showing a relationship between voltages of a common electrode and a pixel electrode in an ideal condition. It is assumed that the voltage of the common electrode is a common voltage Vcom, and the voltage of the pixel electrode may be a positive polarity voltage V+ or a negative polarity voltage V−. The tilts of the liquid crystal molecules only relate to a voltage difference between the common electrode and the pixel electrode. Hence, as long as a difference Vg1 between the positive polarity voltage V+ and the common voltage Vcom equals to the difference Vg2 between the common voltage Vcom and the negative polarity voltage V−, the display units display the same brightness when the positive polarity voltage V+ and the negative polarity voltage V− are input to the pixel electrode. In order to protect the liquid crystal molecules, the positive polarity voltage V+ and the negative polarity voltage V− have to be alternately input to the pixel electrode.
FIG. 2 is a schematic illustration showing a liquid crystal display panel using a dot inversion display mode. When the liquid crystal display panel is driven under the dot inversion display mode, the image quality is better than that of any other display mode. In FIG. 2, each grid represents a display unit, which may be a red, green or blue display unit, wherein the sign “+” represents a display unit with positive polarity, and the sign “−” represents a display unit with negative polarity. The positive polarity voltage V+ is applied to the pixel electrode in the display unit with positive polarity, and the negative polarity voltage V− is applied to the pixel electrode in the display unit with negative polarity. The display method for the dot inversion display mode is such that adjacent display units have different polarities when the (i)th frame is displayed, and the polarities of all display units are switched to different polarities when the (i+1)th frame is displayed.
However, when the common voltage Vcom is drifted, the phenomenon of display flicker may occur. The reason is described in the following. FIG. 3 is a graph showing a relationship between voltages of a common electrode and a pixel electrode in a practical condition. Even though the positive polarity voltage V+ and the negative polarity voltage V− corresponding to the same gray-scale value are input to the pixel electrode, the drifted common voltage Vcom makes the difference Vg1′ between the positive polarity voltage V+ and the common voltage Vcom and the difference Vg2′ between the common voltage Vcom and the negative polarity voltage V− different from each other, and also makes the brightness of the display units different from each other. With regard to the same display unit, although the same gray-scale value is represented, different brightness may occur in the (i)th frame and the (i+1)th frame, thereby causing the phenomenon of frame flicker.
Particularly, when the property of some of the displayed frames meet specific patterns, the phenomenon of frame flicker will be more conspicuous. Conventionally, the displayed frames and the specific patterns are compared first. If the displayed frames have the specific patterns, the display mode of the liquid crystal display panel is switched to a two-line dot inversion display mode in order to ease the phenomenon of frame flicker. The display method for the two-line dot inversion display mode is described in the following. FIG. 4 is a schematic illustration showing a liquid crystal display panel using a two-line dot inversion display mode. When the (i)th frame is displayed, adjacent display units in the same row are alternately arranged as one positive polarity display unit and one negative polarity display unit, and the display units in the same column are alternately arranged as two positive polarity display units and two negative polarity display units. When the (i+1)th frame is displayed, the polarities of all display units are switched to different polarities.
However, the conventional method has the following drawbacks. Since the conventional method for detecting the flicker pattern compares fixed patterns with the displayed frames, the images of the red, green and blue display units have to be separated for comparison. Thus, the required operation load is very high and the required hardware area is also large. It is therefore an important subject to reduce the operation load and the hardware area.